In many cases, spines are a defense mechanism that help protect the animal against potential predators. Because spines are sharp, they can puncture skin and inflict pain and damage which may cause the predator to avoid that species from that point on.

The spine of some animals are capable of injecting venom. In the case of some large species of stingray, a puncture with the barbed spine and the accompanying venom has occasionally been fatal to a human.

Animals such as porcupines are considered aposematic, because their spines warn predators that they are dangerous and potentially toxic.[1] Porcupines rattle their quills as a warning to predators, much like rattlesnakes.[1]

Because many species of fish and invertebrates carry venom within their spines, a general rule of thumb is to treat every injury as if it were a snake bite. Venom can cause intense pain, and can sometimes result in death if left untreated.[2]

On the other hand, being pricked by a porcupine quill is not dangerous, and the quills are not poisonous. The quill can be removed by gently but firmly pulling it out of the skin. The barbed tip sometimes breaks off, but it works its way out through the skin over time.[3]